Incidence of parasitic infection lower when more species present

By Charlie Brennan Camera Staff Writer

Posted:
02/13/2013 07:49:21 PM MST

Updated:
02/13/2013 07:50:12 PM MST

A northern leopard frog is seen with limb deformities caused by a parasitic flatworm. A new University of Colorado study shows that the greater the diversity of amphibian species living in a pond, the less vulnerability frogs, salamanders and toads living there would have against parasitic infection that can cause deformities such as the growth of extra legs.
(Courtesy University of Colorado)

Biodiversity preservation is linked to limiting the spread of disease in a new study unveiled Wednesday by a researcher at the University of Colorado.

Findings from the study, led by Pieter Johnson, an assistant professor in CU's Department of Ecology and Evolutionary Biology, were published in the journal Nature.

The study determined that the greater the diversity of amphibian species living in a pond, the less vulnerability frogs, salamanders and toads living there would have against parasitic infection that can cause deformities such as the growth of extra legs.

By extension, researchers believe their results indicate that biodiversity in larger ecosystems could also afford greater protection against diseases that attack people, such as West Nile virus, Lyme disease and hantavirus.

"At the heart of the study, I think, is really trying to get a better understanding of how preserving or maintaining biodiversity may be beneficial to controlling wildlife disease, or even human infectious diseases," Johnson said.

"I'd say in the last three decades there has been tremendous interest in the value of biodiversity and what it can do in relation to things like crop yields, pest control, water purification, nutrient cycling, biomedicine," Johnson added.

"All of those would be examples of interest in biodiversity and what value it might have for society."

A challenge for Johnson's team was the inherent difficulty in constructing comprehensive studies that might highlight the link between disease transmission and the number of species present in a large ecosystem, due to the high number of living things to be sampled and the wide area those species might inhabit.

The scientists answered that problem by shrinking the habitats being examined.

Over three years, they traveled to hundreds of ponds in California, primarily in the Bay Area south of San Francisco, recording the types of amphibians found there along with the number of snails infected by the pathogen Ribeiroia ondatrae. Snails are an intermediate host used by the parasite in one stage of its life cycle.

The researchers backed up their work in the field with laboratory tests measuring how susceptible to infection each amphibian species is. They also created pond replicas using plastic tubs stocked with tadpoles that had been exposed to a known number of parasites.

Over the course of three years, researchers sampled 345 wetlands and recorded malformations such as missing, misshapen or extra hind legs caused by parasitic infections in 24,215 amphibians. They also catalogued 17,516 snails.

Their results show that ponds with a half a dozen amphibian species had a 78 percent reduction in parasite transmission compared to ponds with only one amphibian species.

The study was co-authored by CU graduate students Dan Preston and Katie Richgels, along with Jason Hoverman, a former postdoctoral researcher in Johnson's lab who now is an assistant professor at Purdue University. It was funded by the National Science Foundation, the National Geographic Society and the David and Lucile Packard Foundation.

Johnson said the findings come at a time when biodiversity is a threatened commodity.

"Many scientists argue we are in the midst of a sixth mass extinction event," Johnson said.

The most recent, 65 million years ago, was the elimination of dinosaurs after a large meteor struck the Earth.

"But this is unique, in its being human-caused. Habitat loss, habitat modification, pollution, are all contributing to some pretty dramatic changes in biodiversity, worldwide.

"If you look among the vertebrates, amphibians are arguably the most threatened vertebrates on the globe. About of the third of them are threatened or declining."

Primary causes of species depletion cited by Johnson are habitat loss, infectious disease, pollution and the influx of invasive species.

There is not unanimity of opinion, however, that biodiversity reduces disease. Bradley Cardinale, an associate professor and director of the Conservation Ecology Program at the University of Michigan, was the lead author in a June 2012 study, also in Nature, which reviewed more than 1,700 peer-reviewed papers on the subject of biodiversity.

Of those, 107 papers examined how biodiversity relates to disease in plants, while another 45 dealt with disease in animals. Cardinale said that 67 percent of the studies supported the contention that biodiversity tends to reduce disease -- meaning that roughly a third did not.

"The bottom line for me is, this is suggestive of it requiring more attention and research," Cardinale said. "I would proceed with caution, with grandiose claims about how important biodiversity is for humans, so far.

Cardinale added, "Pieter's is one case study in a pretty large body of literature that says about two thirds of the time, biodiversity reduces the spread of diseases. If we go around generally advocating for diversity, and how great it is for (limiting) disease, if one third of the time you are wrong and disease goes up, that has important implications for humans."

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